The present invention relates generally to contrast generation for MR imaging and, more particularly, to a method and apparatus of background suppression particularly useful in angiography and perfusion imaging.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
MR angiography and arterial spin label perfusion are MR imaging techniques that have been shown to be probative in the imaging of the vasculature and hemodynamic state of the brain for evidence of a number of pathologies and diseases including, but not limited to tumors (both pre- and post-operative), vascular malformations, various dementias including Alzheimer's disease, and epilepsy to name just a few. In the imaging of blood perfusion, tissue exogenous and endogenous tracers are commonly used to produce differential contrast. One example of an exogenous tracer or contrast agent is gadolinium-diethylenetriaminepentaacetic acid (GD-DTPA) which, when introduced to a patient, modifies the relaxation of protons in the blood, including producing a shorter T2* relaxation time. Endogenous tracers, on the other hand, depend upon the ability to generate contrast from specific excitation or diffusion mechanisms resident in the patient. For example, in “black blood” perfusion, inflowing spins or protons in the blood are labeled and, as such, function as a contrast agent. The labeled spins outside the imaging volume perfuse into tissue thereby resulting in a drop of signal intensity that can be measured over time to provide quantitative measurements of the time course of events such as time to peak, contrast enhancement ratio, and slope of the first pass contrast enhanced MR images.
Blood oxygen level dependent (BOLD) and “functional MR” imaging are examples of MR perfusion imaging techniques that are used to image the brain. Both of the aforementioned imaging techniques rely on the differential contrast generated by blood metabolism in active areas of the brain. Inflow of blood and the conversion of oxyhemoglobin to deoxyhemoglobin increases the magnetic susceptibility in a localized area as well as induces signal loss by reducing the T2* relaxation time of “tipped” spins. As such, this allows areas of high metabolic activity to produce a correlated signal. Generally, a control or mask image is acquired prior to application of a particular stimulus for passage of the exogenous or endogenous contrast agent through the imaging volume. Following acquisition of a series of MR perfusion images, the mask or control image is subtracted from each of the MR perfusion images after image reconstruction. The resulting images therefore show the differences between pre-stimulated tissue and post-stimulated or perfused tissue.
As one skilled in the art will appreciate, the creation of control or mask images as well as the subtraction of the mask or control images from the MR perfusion images increases overall scan time, decreases patient throughput, and increases the likelihood of patient induced motion artifacts.
MR angiography (MRA) is an imaging technique similar to perfusion imaging in that signal from fluid, i.e., blood or cerebral spinal fluid (CSF), is used as the basis of contrast in a reconstructed image. Time-of-flight (TOF) MRA is an MRA imaging technique that relies on the tagging of blood in one region of the body and detecting in another. As such, when the tagged blood enters a particular imaging volume, the tagged blood will provide contrast differentiation from the surrounding stationary tissues. Generally, tagging of the blood is accomplished by spin saturation, inversion, or relaxation to change the longitudinal magnetization of moving blood. One skilled in the art will appreciate that the penetration of tagged blood into the imaging volume depends on the T1 relaxation time of the blood, its velocity, and direction of flow. Like MR perfusion imaging, the effectiveness of MRA is largely predicated upon the degree of contrast achieved between the stationary or static background tissue and the inflowing fluid. That is, for the reconstructed image to be generally diagnostically valuable for the identification and detection of pathologies, detectable contrast between the inflowing fluid and the background tissue must be present. Heretofore, contrast differentiation has been primarily improved with black blood nulling wherein readout occurs when the relaxation time of blood crosses its null point, mask image subtraction, or through the use of exogenous contrast agents.
Notwithstanding the advancement of black blood nulling, mask image subtraction, and exogenous contrast agents, each of the aforementioned techniques increases scan time and/or depicts inflowing blood or other fluid as a signal void. It would, therefore, be advantageous to design a method and system of MR imaging with background suppression and data acquisition from inflowing fluid with reduced scan time.